President Obama has tried to inject a deliberate note of realism into his new strategy for Afghanistan – at least what we know of it so far. On the eve of departure for the G20 and Nato summits in Europe, he has announced he will send 4,000 extra US troops to the country, a total US reinforcement of 21,000 this summer, and bring in a range of expensive aid and reconstruction programmes for Afghanistan and Pakistan, the "AfPak" region in the new American strategic jargon.

This concept could turn out to be a bit of a two-headed monster for the allies, particularly the UK. The main thrust of the Obama strategy is that the fight against al-Qaida and the senior command of the Taliban has to be taken into northern Pakistan from Baluchistan to the playground of the militants in Kashmir.

They will expand the war by proxies, air strikes and drones into the territory of a friendly country, Pakistan, albeit one that is in a state of almost chronic fracture. Increased military activity by America and its allies could make things even more unstable – and throughout the country.

Obama has decided that al-Qaida is the main threat to American interests and so it must be the main target. For Afghanistan, he seems to have reduced the ambitions of his predecessor George W Bush to more modest proportions. Instead of building a model western-orientated democracy and market economy, the aim now is to build enough security in enough of Afghanistan in order to get by.

There will be much less talk of eradicating the narco economy and rubbing out the traffickers and warlords, and more of getting the Afghans to run their own security in such a way that they don't upset the neighbours. The Afghan army and police are to be doubled in a couple of years. The extra 2,000 British troops offered by General Sir Richard Dannatt in the Times today – though not cleared by Downing Street – would be involved in the training and reconstruction mission as much as fighting the Taliban, one presumes.

With the Americans now providing nearly 60,000 troops, and with the modest increases now on the cards from countries like Britain, France, Italy and Germany, the international troop presence will tip towards the 110,000 mark – a figure with an ominous echo from the recent past. That was the number the Russians had in Afghanistan at the peak of their ill-fated occupation in the 1980s.

The Russians have been warning the international allies of ISAF (International Support and Assistance Force) that you can't fix Afghanistan with this shape of military presence. They have even agreed to host a conference on security in Afghanistan, which would also embrace the Iranians, who are equally worried about their eastern neighbour descending into deepening Taliban/al-Qaida orchestrated chaos.

Obama may find difficulty in the coming days in convincing his Euopean allies that his AfPak plan can work. The American military has made it clear they don't want much more help from European forces, because they're not up to the job. American forces are now so technically superior in terms of airpower, surveillance, targeting and communication that it is almost impossible to work with even the best European forces, such as the British and French, because they are so inferior and deficient in equipment such as numbers of aircraft and drones and communications.

So the fighting campaign will be increasingly American-commanded and American-executed – and working to an American script. This is why extending the war into northern and eastern Pakistan is so risky for the British, in particular. Britain has at least 1,300,000 Kashmiri residents – whereas as America has only about 30,000. "We don't want the fighting and tensions of the Khyber and Kashmir fought out in Rotherham or Bradford," a British politician with a northern constituency told a French colleague recently.

The notion of AfPak as one regional theatre of war touches on an issue of domestic security for the UK, one which the US itself does not have and seems reluctant to recognise could be a major concern for an ally.

There is also a question about the growing "surge" mythology that accompanies General David Petraeus, the Cent Com commander and Obama's military point-man on Afghanistan. The mythology states that the surge of 35,000 extra troops carried out by Petraeus turned disaster into triumph in Iraq in under two years. Violence levels are down, prime minister Maliki is stronger than ever, and the Americans can pull back next year, and pull out altogether by Christmas 2011. So why can't the same recipe work for Afghanistan?

First, the terrain both human and physical is very different and more complex. Second, the extension of a conflict onto the territory of a friendly, and highly fractious, allied country has a very bad resonance from the recent past. The expansion of the Vietnam war into Cambodia and Laos, for example, made things much worse.

Third, the jury is still out on the success or failure of the surge in Iraq. The surge brought a drop in violence, sure. But several of the militias decided to declare a ceasefire until the Americans went away. There has been very little of the political and social reconciliation that was supposed to accompany the military operation. Iraq, in many areas, such as Mosul and Kirkuk, is still bitterly divided, and some resident diplomats and analysts fear that a further bout of civil war could be on the way.

So Obama may well have to rethink his strategy on Iraq, as well as Afghanistan, well before his first term is up.